I have been cycling a small tank for a week, without fish. It is a dirt/gravel layered substrate. The phosphate was very high and algae began to grow rapidly on rocks and moss within 3 days.

I have over the last three days been dosing with ammonium chloride to achieve a measured level of 5 pm. The first addition showed an unexpected response. The algae simply disappeared within 2 hours leaving only a white filament where the blooms were. The plants and mosses weren't affected.

The next day I measured the ammonia and it had dropped to 1 ppm. The nitrites were high and the nitrate at a low reading of 1 ppm. Phosphate still very high. I watched the tank over the next day and the algae bloom returned. A further dosing of ammonium chloride to 6 ppm didn't affect the algae. The Nitrites were even higher and the phosphate essentially the same high reading. Nitrate still hasn't appeared at a high level, now somewhere between 1 and 5 ppm.

The cycling seems to be going as expected but I can't find anything mentioned about the selective algaecide effect of the initial high ammonia dosing. I would be grateful if someone has an explanation or similar experience. I'm not concerned by the algae at this stage and I will add a phosphate capture when the tank has cycled to sequester the phos from the dirt layer in the substrate.